The study of genetics is incomplete without understanding the laws put forth by Mendel. After his experiments of monohybrid and dihybrid crosses, Mendel concluded that these are certain factors which are passed from one generation to the other. But he explained this better with the help of his first law in genetics. The first law in genetics is called the law of segregation.
To understand this law, we first need to understand the concept of alleles. We know that genes, in simple terms, are the units of heredity that carry genetic information. Alleles are nothing but the different forms of genes.
Let's take a simple example to understand this. Assume we have these two chromosomes. Here we have genes for the character height. Can you tell me which genes could possibly be present here? We can have either both capital TT or both lowercase tt and we can even have one capital and one lowercase tt as the gene set.
Now here the individual T, that is the single gene, is nothing but the allele. So the alternative form that is either dominant or recessive is called allele. In simple words, one single T or T is an allele. Now that we know what alleles are, let's try to understand the law. There are different definitions you will find for this law, but let's pick the simplest one.
It says that during gamete formation, the alleles for each gene segregate from each other such that each gamete formed carries only one allele for each gene. It may still sound a bit confusing. To understand this, let's take a simple example.
Let's assume we have this single cell with only two chromosomes in it. The chromosomes have a set of genes CC for the skin colour in an individual. Now to understand the law, let's break it into parts.
The first part says, during gamete formation. This is the process of meiosis. It is the type of cell division which gives us four cells with half the number of chromosomes. The second part says the alleles of each gene. So here, C and C is what we are referring to.
That means one allele from the gene set. Now what's the next part? The next part of the statement is SEGREGATE from each other. This means the alleles will separate. They will not remain together in the process.
And now comes the last part. It says that one gamete carries only one allele for each gene. Now this part means that at the end of the meiosis process, we will get four cells such that each cell will have only one allele in it. So here, two cells will have capital C and the other two cells will have the lowercase c allele.
Thus, each gamete gets only one allele. To summarize, we can say that the alleles for a gene segregate during gamete formation. To be honest, this law sounds very obvious to us now. It's like having a big book with many chapters in it and then we make smaller books for each chapter.
It is obvious that the smaller books will have the content resembling the content of the big book. This law sounds obvious also because we know the process of meiosis. But nobody knew this back in the earlier times.
Hence, it was hypothesized by Mendel and then it was accepted as a law later. So what's the next law? It's the law of independent assortment. Does this mean that the genes assort independently? Or is there anything else that the law specifies?
Let's find that out in the next part! There is one common thing among sea waves, dust particles and a dice rolled on the floor. Any guesses what that common thing could be? It's randomness!
Randomness is commonly found in nature and in several things around us. But can you state an example of randomness that occurs inside us? Door- body seems to be organized, there are a few processes that have no fixed pattern. One of the processes is that of the alignment of chromosomes on the metaphase plate. Let me elaborate.
Imagine this to be a cell that's about to undergo division. For now, let's assume it's undergoing meiosis. We know that during cell division, a stage named metaphase is reached where the chromosomes align themselves on the equatorial plane. In other words, on the metaphase plate. So, can you tell me how these chromosomes will align before they line up on the plate?
Will all the red chromosomes from each pair line up on one side or will they align in an alternate manner? Or maybe like this? Or will the alignment be like this?
There are numerous ways in which this can happen. And this is random! There is no fixed pattern. or a principle of sequence that the chromosomes follow when they align. Now if the chromosomes get separated randomly, it's obvious that the genes present on them will also get separated randomly.
So let's say this pair has the genes capital A and lowercase a, this pair has the gene set capital B and lowercase b, while this one has capital C and lowercase c on it. We are considering all the sets to be heterozygous in order to understand the concept better. So when the chromosomes get separated in the respective cells, it's obvious that the genes will assort independent of each other.
There is no thumb rule that all the dominant alleles will perpetuate in one cell and all the recessive alleles in the other. They can assort independent of each other. This is exactly what Mendel had to say in his second law of genetics. It's named the law of independent assortment. The law says that genes for the different traits assort independent of each other during gamete formation.
Let's understand this with the help of this cell. The first part says, genes for different traits. This means we are addressing the genes which represent diverse traits.
So here, We have the gene sets AA, BB and CC on these respective chromosomes. These are the genes for different traits. Now the second part states, Assort independent of each other.
This means the genes will not have a fixed pattern to follow when they segregate in the cells. So here, the alleles of the gene set AA will sort independent of the other two gene sets. It's not mandatory that all the dominant alleles will get assorted in the cells together. So the allele capital A can get assorted with any allele from the gene set BB and CC. And that's the reason why we get the gametes with different possible combinations at the end.
To quickly review, the assortment of alleles is purely random and does not have a predictable and fixed pattern during the process of gamete formation. That is what the law says! There can be more possibilities of obtaining different combinations depending upon how the chromosomes align themselves on the metaphase plate. Now tell me, which type of a cross will help us understand this law better? A mono-hybrid cross or a dihybrid cross?
Think about it! Let me help you with this! If it's a mono-hybrid cross, then there will be no independent assortment of the different alleles. And why is that so? That's because we are considering only one character at a time.
So we will be dealing with a single gene set. And thus, this law is best understood when there are more than one character studied at a time. This is nothing but the dihybrid cross or higher crosses which involve more characters. Now that we are well versed with the first two laws of genetics, let's have a look at the third law in our upcoming part.
A scene of lush green trees covered with yellow or brown mesh is not new to us. We have come across it many times. Any idea what this mesh is all about? These are also plants, parasitic plants to be precise. Being parasitic, They derive all the nutrients from the host and grow to form a huge structure.
These plants are like creepers and overpower the huge trees as they grow. This is one of the best examples for understanding the fact that dominance is not a matter of size. Rather, it's never affected by size.
It's the nature of the dominating component that matters. Let's take our genes for example. We know how small genes and their constituent alleles are.
Yet, there is one form which dominates the other. We are familiar with the concept of dominant genes and recessive genes. But this wasn't known back then. Not even to Gregor Mendel! That is the reason why he put forth the concept of dominant and recessive alleles with the help of this law.
This law is called the Law of Dominance. After the Monohybrid Cross, Mendel concluded that a few genes are dominant while the others are recessive. Let's have a look at what the law has to say!
The third law of genetics, known as the Law of Dominance, states that some alleles are dominant while others are recessive. An organism with at least one dominant allele displays the effect irrespective of the presence of the recessive one. The law is actually very simple. Let's take an example and understand it. Let's consider a cross of a tall and a dwarf plant.
The first part of the law says that some alleles are dominant while others are recessive. Here, can you tell me why is this plant tall? That's because it has both the dominant alleles.
We represent it with the capital letters TT. Similarly, this is a dwarf plant as it has both the recessive alleles denoted by TT in the lower case. Now the next part of the law states that An organism with at least one dominant allele displays the effect irrespective of the presence of the recessive one. So what does that mean?
Here, on crossing the two, we get these plants in the F1 generation. The phenotype of all these plants is tall. And what is their genotype? As we can see, they are all heterozygous and with one dominant and one recessive allele. Doesn't the tall phenotype clearly indicate that the dominant allele has masked the presence of the recessive one?
That's right! That's what the second part of a law says! The presence of a single dominant allele is enough to express the trait phenotypically.
And what if both the genes are dominant? It's obvious that the phenotype will be dominant! The law states at least one dominant allele, which means if one or even both the alleles are dominant, then we get the respective dominant phenotype. Now let's cross this F1 generation to check whether we have understood the law correctly. The cross of these F1 hybrids gives us these offsprings in the F2 generation.
Can you help me with the phenotype and the genotype of these offsprings? Here we get three tall plants and one dwarf plant. Genotypically, There is one plant that is homozygous for the dominant trait. There are two plants which are heterozygous.
And this last one is again homozygous but for the recessive trait. So can we relate these results with our law of dominance? There are two types of alleles.
One dominant and one recessive. This is what the first part says. And the second part says that at least one dominant allele displays the effect irrespective of the presence of the recessive one. So as we can see here that these three plants are tall because they contain the allele T for tall trait. This one has both the dominant alleles and these two have one allele for the dominant trait.
Its presence masks or suppresses the effect of the recessive allele. So this was about the third law of genetics called the law of dominance.